1. Field of the Invention
The present invention relates to an insulated conductor pair and a guide cable using the same.
2. Description of the Prior Art
Communication between a movable body such as a vehicle moving along a railway track or road, and a ground station conventionally employs a so-called guide or leaky cable that is laid along the track or road. Bidirectional communication is effected to transmit and receive information by coupling an electromagnetic field leaking in a suitable degree from this guide cable with an antenna provided on the movable body.
As another application, an electromagnetic field may be caused to leak intermittently from the guide cable rather than continuously. Distances between the positions where the electromagnetic field leaks and the ground station may be accurately measured and gauged. Then, the position of the movable body from the ground station can be monitored accurately based on the reaction of the movable body to a signal transmitted from the ground station or based on a phase difference of a signal transmitted from the movable body and received by the ground station via the guide cable. Because of the above application, the guide cable is also called a movable body position detecting guide cable.
The above guide cable is, as shown in FIG. 8, formed such that two insulated wires 3, 3' each comprised of a conductor 1 and an insulating coating 2 are twisted in one direction to form an insulated wire pair 4. Electromagnetic field coupling portions (opened portions) 5, where the insulated wires 3, 3' are spaced from each other in parallel, are formed at intervals along the length of the insulated wire pair 4. The thus formed guide cable 7 is usually called a twisted pair type guide cable.
As shown in FIGS. 9(A) and 9(B), the guide cable 7 is constructed such that a guide pair formed by the insulated wire pair 4 arranged on a base plate 8 is pressingly taped together with an interposition member for holding the configuration of the guide pair and for protecting the guide pair against an external force. The taped guide pair is further surrounded by an outer coating made of a nonmetallic and nonmagnetic (for an efficient leakage of an electromagnetic field) synthetic resin such as polyolefin or vinyl chloride.
Portions of the guide cable 7 at the opposite sides of the electromagnetic field coupling portions 5 where the insulated wires 3, 3' are twisted are electromagnetic field uncoupling portions 6, which have a property of leaking an electromagnetic field in a very small degree as compared with the electromagnetic coupling portions 5 by the twist effect of the insulated wires 3, 3'.
Accordingly, as shown in FIG. 10, when an antenna 9 of the movable body moves along the guide cable 7, an antenna coupling output becomes larger when the antenna 9 approaches the electromagnetic field coupling portion 5 while becoming smaller when the antenna 5 approaches the electromagnetic field uncoupling portion 6.
Facilities for manufacturing the insulated wire pair 4 of the guide cable 7 are usually as follows. Reels on which the insulated wires 3, 3' are wound up are mounted on a rotary feeding apparatus. The insulated wires 3, 3' let out from the respective reels are guided by wire guides to a twisting die while the rotary feeding apparatus and the respective reels are rotated. The insulated wire pair 4 is withdrawn from the twisting die by being held between belts of a withdrawing apparatus while stopping the rotation. In this way, the insulated wire pair 4 twisted in one direction is formed and taken up by a drum of a take-up apparatus.
The insulated wire pair 4 taken up by the drum of the take-up apparatus is let out from this drum by a suitable length, and the insulated wires 3, 3' are untwisted at intervals along the length of the insulated wire pair 4 and arranged in parallel. By widening the spacing between the untwisted portions of the insulated wires 3, 3' in parallel with each other, the electromagnetic field coupling portion 5 is provided.
However, the conventional twisted pair type guide cable 7 using the insulated wire pair 4 requires large manufacturing facilities since two insulated wires 3, 3' are twisted to manufacture the insulated wire pair 4. Further, to improve the accuracy of twisting intervals of the insulated wires 3, 3', which directly influences the antenna coupling output, the manufacturing facilities need to be controlled with high precision. Accordingly, there are problems of the large facilities and high running costs.
An operation of providing the electromagnetic field coupling portions 5 by: untwisting the insulated wires 3, 3' at intervals along the length of the insulated wire pair 4, arranging the untwisted portions in parallel, and widening the spacing between the untwisted portions in parallel with each other is a very difficult operation which still requires a lots of time and labor despite the use of large-scaled machines and tools. It is also difficult to accurately space the insulated wires 3, 3' in parallel with each other in the electromagnetic field coupling portions 5, and the insulated wires 3, 3' are nonuniformly twisted and arranged in boundary portions between the electromagnetic field coupling portions 5 and the electromagnetic field uncoupling portions 6. As a result, a highly precise and stable degree of electromagnetic coupling is unlikely to be obtained.
Particularly, since the twisted insulated wire pair 4 has a circular cross section and is likely to rotate about its axis, the twisting intervals may vary due to the rotation of the insulated wire pair 4 about its axis upon being subjected to an external force while the insulated wire pair 4 is assembled into the guide cable.
Further, since the insulated wires 3, 3' having a circular cross section are used, the guide cable 7 has a large thickness (height). Accordingly, such a guide cable 7 cannot be laid in a place which needs to be thin, such as a floor surface, a wall surface or a narrow cable passage portion. Additionally, it is not possible to completely eliminate the leakage of the electromagnetic field by electromagnetically screening only portions corresponding to the electromagnetic uncoupling portions 6 of the guide cable 7.
The present invention was developed in view of the above problems, and an object thereof is to provide an insulated conductor pair which can be manufactured easily in inexpensive facilities and with which a highly precise and stable degree of electromagnetic coupling can be obtained. An object of the invention also is to provide a guide cable using such an insulated conductor pair.